Subsea pipelines form an integral part of the infrastructure associated with offshore oil and gas developments. These pipelines fulfill a range of functions from linking extraction wells to other subsea infrastructure to transporting products onshore, or to a central processing facility. Ancillary pipelines may also be present for gas or water injection to the reservoir or transporting additives. Pipelines are typically installed directly onto the seabed and, in the absence of significant drivers to undertake burial operations, they may remain on the seabed for the remainder of their design life. This is typically the case for deepwater developments. Subsea pipelines are subjected to a wide range of load cases including, self weight, installation loads, thermal and pressure driven expansion and hydrodynamic loading. Design of pipeline systems to accommodate these load cases requires an understanding of pipe-soil interaction. This thesis reports the results of a research study investigating pipe-soil interaction on a clay seabed, as relevant to the design of subsea pipeline systems. This study has utilised numerical analysis techniques based on the finite difference code FLAC to investigate a range of problem definitions. These problem definitions include pipelines subject to both vertical loading (V) and combined vertical and horizontal (V-H) loading. Factors such as variation in interface conditions, large strain and large displacement effects, soil unit weight effects and variation in shear strength conditions were considered in these problem definitions. Reliability based analysis techniques have also been used to investigate both V and V-H loading problem definitions. The analyses and investigations undertaken as part of this study generally achieved the following; reproduction and validation of earlier research with additional interpretation, extension of problem definitions to deeper pipeline embedment depths and investigation of pipe-soil interaction problem definitions that have not previously been considered. Reliability based analysis techniques have also provided some interesting insights into the impact of soil shear strength variation as well as providing a fundamental link between safety factors and probability of failure. Application to design practice of this, and similar studies, has been considered as part of this thesis and potential areas for future research have also been suggested.

Horizontal directional drilling is increasingly used to install pipes without costs and disruptions associated with conventional ‘cut and cover’ installations. This technique, which was developed by industrial innovators, feature complex soil and pipe response which is not well understood. The success of this operation depends on knowledge of the pulling forces applied, level of ground disturbance, ground expansion or fracture from mud pressure, and the effect of the pulling operation on the pipes. Tensile stresses in the pipe vary with time during and after installation, and along the pipe. This applies especially to polymer pipes where the stresses during insertion and those over the service life of the pipe may influence its performance. The main objective of this study is to model the short term and long term response of pipes installed using horizontal directional drilling and to investigate the effect of the time dependent behaviour of polymer pipes, as well as other installation variables on the performance of the pipe during and after installation. The mechanical behaviour of high density polyethylene used to manufacture a significant portion of pipes installed using horizontal directional drilling is investigated and two sophisticated constitutive models are developed to simulate the time-dependent behaviour of high density polyethylene. The interaction between the pipe and the surrounding soil during horizontal directional drilling installations is also investigated and modelled. A FORTRAN algorithm is developed to calculate the short and long term response of elastic and polymeric pipes installed using horizontal directional drilling. The program uses the HDPE constitutive models as well as the pipe-soil interaction model developed in the study. After evaluation, the developed program is employed in a parametric study on the sensitivity of short term and long term pipe response to different parameters, including the effect of overstressing the pipe during installation. As Multiaxial modeling is necessary for accurate analysis of some applications including the swagelining method, a uniaxial constitutive model developed in the current study is generalized to a multi-axial model that can simulate the response to biaxial stress-strain fields. The multi-axial model is implemented in a finite element code and its performance in simulating multiaxial stress-strain fields is evaluated. / Thesis (Ph.D, Civil Engineering) -- Queen's University, 2008-06-12 18:03:43.501

Offshore on-bottom pipelines are subjected to cycles of thermal and pressure-induced axial expansion, which can cause them to buckle laterally. For an elegant and cost-effective solution, lateral buckling is allowed in a controlled manner. Of the various design parameters, the soil resistance has the greatest associated uncertainty. Previous studies of lateral pipe-soil interaction have used laboratory model tests and continuum-based numerical methods. However, they are economically and computationally expensive, and have mostly been restricted to pipes on undrained clay. To overcome this limitation, this thesis introduces the distinct element method (DEM) as a novel numerical tool for the study of lateral pipe-soil interaction for partially embedded offshore pipelines on sandy seabeds. The DEM directly models the particulate nature of sandy soils, allowing large displacements of discrete bodies and providing insights into the mechanics of the soil at a particle level. Pipe{soil interaction is studied by DEM analyses through four separate research stages: (i) mechanical characterisation of the soil, (ii) specimen preparation and pipeline implementation, (iii) small displacement pipe loading tests and (iv) large displacement pipe loading tests. The soil is modelled as an assembly of spherical particles exchanging contact forces, energy and momentum when they interact. At the microscopic scale, a novel moment-relative rotation contact law is introduced to account for the irregular shape of real sand grains. At a macroscopic scale, the mechanical behaviour of the sand is calibrated using experimental triaxial test data. Additional work includes the numerical preparation of a soil assembly and the implementation of a pipeline object in the open-source DEM code Yade. A novel specimen preparation technique is developed to assemble a homogeneous sample at a desired relative density. The pipeline is implemented as a cylindrical body with a continuously curved surface and a specific mass. Small displacement loading tests are performed, with a segment of the pipeline interacting with a 3D prismatic soil domain, replicating plane strain conditions. The influence of particle size, domain thickness, loading velocity and damping are investigated. The findings provide valuable recommendations for performing DEM simulations of this problem, balancing numerical accuracy and computational effort. Large displacement loading tests are performed to validate the DEM approach and to obtain detailed insights into the nature of the pipe-soil interaction. Monotonic vertical and lateral loading simulations are quantitatively compared with laboratory results. To replicate realistic loading conditions of the pipeline on the seabed, cyclic large displacement tests are also performed. Both the monotonic and the cyclic tests show a good level of agreement with experimental results obtained in previous research. Moreover, the numerical analyses provide insights into the evolution of particle motion and the failure mechanism within the soil.